The need to distinguish between odd and even fields arises when an auxiliary image is simultaneously displayed as an overlay to a primary image which typically fills the display area. For example, a picture-in-picture (pix-in-pix) display or the display of a function menu. The overlay and primary fields are asynchronous, making it necessary to distinguish between odd and even field types to assure that the overlay signal is stable with respect to the main field in order to maintain proper interlace and avoid jitter of the auxiliary display.
A known technique of distinguishing between the odd and even fields of a composite video signal includes monitoring the horizontal and vertical blanking signals. The blanking signals are provided by a deflection processor and provide noise immunity. These signals provide time references with respect to the main display which are jitter free because the main display also uses these signals. When the horizontal and vertical deflection signals are used for overlay processing, a problem commonly known as "threshold straddling" frequently arises. Odd and even field type are determined by detecting the phase relationship of the horizontal and vertical blanking signals. A phase exists at which it is impossible to definitely distinguish whether a field should be called even or odd. This is the threshold phase and when the deflection signals are in the vicinity of this phase there is a potential for threshold straddling and vertical jitter of the overlay display can ensue.
There are several techniques which can be used to avoid threshold straddling. One way is to restrict or center the horizontal/vertical blanking relationship. In the NTSC signal the timing of the horizontal and vertical sync pulses is well controlled. However, modern deflection systems produce horizontal and vertical blanking signals which typically have a random phase relationship. Adding to the difficulty of using this phase relationship is the fact that the relationship typically can change from day-to-day, receiver-to-receiver or channel-to-channel. It is theoretically possible to avoid threshold straddling by careful system design, the addition of an alignment circuit or by maintaining tight system tolerances. However, these methods are both impractical and expensive with today's deflection techniques.
A second technique which is more advantageous and prudent is to design an overlay processor which is immune to threshold straddling. This results in an overlay processor which is less expensive and which can be used in a variety of deflection designs. Implementation of this approach requires two functions. First, a means for sensing when the phase relationship is in the vicinity of the threshold. Second, circuitry for shifting the phase relationship away from the vicinity of the threshold. The present invention fulfills both of these requirements. The second requirement can be fulfilled in a number of ways. For example, by delaying either the horizontal or the vertical blanking signal with respect to the other. The invention uses a dual mode field type detector approach. With the dual mode detector approach, when the horizontal/vertical phase relationship of the blanking signals is in the vicinity of the threshold value, a switch-over signal is produced and the phase relationship is moved away from the threshold value. This approach also identifies each field as an odd or an even field. A detector which identifies field type in a manner similar to the invention, but which does not sense when the phase relationship is in the vicinity of the threshold value, is described in U.S. Pat. No. 5,025,496.